Multiple-layer, cook-in film

ABSTRACT

Disclosed is a multiple-layer cook-in film from which packages such as bags or casings can be made. The films have a first food contact layer that bindingly adheres during cook-in to a contained meat product. The first layer comprises a polyamide composition, which is preferably (I) a polyamide or (II) a blend of a polyamide and (i) polyethylene, or a blend of a polyamide and (ii) a copolymer of an alpha-olefin having the formula RHC═CH 2  wherein R is H or C 1  to C 8  alkyl and an alpha,beta-ethylenically unsaturated carboxylic acid.

BACKGROUND OF THE INVENTION

This invention relates generally to thermoplastic films suitable forcook-in packaging, and more particularly to cook-in food films havingfood contact surface characteristics which promote binding adherence toa contained food product during cook-in. The films of the inventionexhibit improved heat seal strength as compared to known films having aSurlyn food contact layer that bindingly adheres during cook-in to acontained meat product.

The food packaging industry needs a packaging film from which bags andcasings can be made which are of improved structural soundness such thatthey may be fully characterized as cook-in. Further, a precooked foodproduct attractively packaged inside the film within which it wasprecooked is desirable. The term "cook-in" as used herein is intended torefer to packaging material structurally capable of withstandingexposure to cook-in time-temperature conditions while containing a foodproduct. Cook-in packaged foods are essentially foods cooked in thepackage in which they are distributed to the consumer and which may beconsumed with or without warming. Cook-in time-temperature conditionstypically refer to a long slow cook, for example submersion in hot waterat 55°-65° C. for 1-4 hours, and such conditions are representative ofinstitutional cooking requirements. Submersion at 70°-100° C. for up toabout 12 hours probably represents the limiting case. Under suchconditions, a cook-in packaging material should maintain seal integrity,i.e. any heat sealed seams should resist being pulled apart duringcook-in. As a corollary, the film is heat sealable to itself.Additionally, the packaging film is substantially conformable to thecontained food product. Preferably, this substantial conformability isachieved by the film being heat shrinkable under these conditions so asto form a tightly fitting package. In other words, in an advantageousembodiment, the film is heat shrinkable under these time-temperatureconditions, i.e. the film possesses sufficient shrink energy such thatsubmerging the packaged food product in hot water will shrink thepackaging film snugly around the contained product, representatively upto about 55% monoaxial or biaxial shrinkage.

Also, the film should have food product adherence to restrict "cook-out"or collection of juices between the surface of the contained foodproduct and the food contact surface of the packaging material duringcook-in, thereby increasing product yield. More particularly, in thetypes of multi-layer films wherein the first "sealing and food contact"layer is of the type of material that bindingly adheres to a containedfood product during cook-in, this first layer may alternatively bereferred to as the "adhering layer". As used herein, the term "adhere"is intended to mean that the food contact surface of the film bondsduring cook-in to the contained food product to an extent sufficient tosubstantially prevent accumulation of fluids between the film and thecontained product.

A heat-shrinkable, cook-in film is described in U.S. Pat. No. 4,469,742(1984) to Oberle et al. This patent relates to a cook-in shrink filmthat includes a first "sealing or food contact" layer of nonlipophillicpolymeric material having a softening point greater than that of thefollowing shrink layer; a second or shrink controlling layer, meltbonded to the first layer, of an ethylene homopolymer or copolymer; athird or adhesive layer, melt bonded to the second layer, of achemically modified polyethylene being irradiatively cross-linkable andhaving functional groups with a relatively strong affinity for thefollowing barrier layer; a fourth or barrier layer, melt bonded to thethird layer, of a hydrolyzed ethylene-vinyl acetate copolymer; a fifthor adhesive layer as in said third layer, melt bonded to the fourthlayer; and a sixth or abuse layer, melt bonded to the fifth layer. Inone embodiment, the first "sealing and food contact" layer is a metalsalt neutralized copolymer of an olefin and a carboxylic acid,representatively Surlyn (™). This patent explains Surlyn is the type ofmaterial that bindingly adheres to a contained meat product duringcook-in. Thus, this Surlyn layer also functions as a protein-adheringlayer. The patent also describes a method for making the film includingfull coextrusion and selective irradiation and orientation.

In the conventional method of manufacturing heat shrinkable film asdescribed in the Oberle et al patent, a tubular orientation process isutilized wherein a primary tube of the film is biaxially oriented bystretching with internal pressure in the transverse direction and withthe use of pinch rolls at different speeds in the machine direction.Then the stretched bubble is collapsed, and the film is wound up asflattened, seamless, tubular film to use later to make bags, e.g. eitherend-seal bags typically made by transversely heat sealing across thewidth of flattened tubing followed by severing the tubing so that thetransverse seal forms the bag bottom, or side-seal bags in which thetransverse heat seals form the bag sides and one edge of the tubingforms the bag bottom. Such bags are typically used by placing the foodproduct in the bag, evacuating the bag, either heat-sealing the bagmouth or gathering and applying a metal clip around the gathered mouthof the bag to form a seal, and then immersing the bag in hot water atapproximately the same temperature at which the film wasstretch-oriented, typically about 160° to 205° F. (61° to 96° C.), hotwater immersion being one of the quickest and most economical means oftransferring sufficient heat to the film to shrink it uniformly.Alternatively, the bag may serve as a liner of a cooking mold.

Also of interest is a plastic, adhering cook-in package such as thecasing described in U.S. Pat. No. 4,606,922 (1986) to Schirmer, relatingto a method for enhancing yield of a cook-in packaged meat product thatincludes first providing an adhering cook-in container including aflexible thermoplastic envelope being substantially conformable to acontained meat product and having an inner meat-contacting surface of aselectively irradiated ionomer of a metal salt neutralized copolymer ofethylene and acrylic acid or meth-acrylic acid, then conforming thecontainer about a selected meat product and cooking the packagedproduct, whereupon the inner surface of the envelope bonds to the meatproduct substantially to prevent cook-out of fluids. Representatively,the ionomer of the inner binding surface is Surlyn, and a typical casingor envelope is of the structure: nylon 6/adhesive/Surlyn.

Also of interest is a flexible plastic, adhering cook-in packagedescribed in U.S. Pat. No. 4,411,919 (1983) to Thompson, relating to amethod for enhancing yield of cook-in packaged meat product, comprising:(a) providing an adhering cook-in package comprising a flexible plasticcontainer being substantially conformable to a selected meat product andhaving an inner meat product contacting surface of polymeric olefinhaving been subjected to an energetic radiation surface treatment in thepresence of oxygen sufficient to cause said inner surface to adhere tothe meat product during cook-in, said container having been formed fromhot blown tubular film; (b) conforming said package about a selectedmeat product; and (c) cooking the packaged product, whereupon said innersurface adheres to said meat product to substantially prevent cook-outof fluids therefrom.

Also of interest is the film described in U.S. Pat. No. 4,303,711 (1981)to Erk and Korlatzki, which relates to a tubular film consisting ofbiaxially stretched plastic material for packing and casing paste-typefoodstuffs that either, after packing, are heated or are packed in a hotfluid state, comprising a mixture of approximately 50-99 parts by weightof at least one aliphatic polyamide having a glass transition point inthe dry state of at least 48° C., and a glass transition point aftermoisture absorption of 3° C. or less and approximately 1-50 parts byweight of one or more members of the group consisting of an ionomerresin, a modified ethylene/vinyl acetate acid copolymer and a modifiedpolyolefin. Another patent to Erk and Korlatzki is U.S. Pat. No.4,601,929 (1986), relating to a single layer of polyamide film forpacking and casing foodstuffs in paste form, especially foodstuffs thatare packed when hot or are subject to heat treatment after packing.

Also of interest is the film described in U.S. Pat. No. 4,568,580 (1986)to Ghiradello et al, relating to an article of manufacture for packagingfood products comprising: (a) a first film section, said section havingat least one surface comprising a copolyamide obtained by randomcopolymerization of precursor monomers of at least two differentpolyamides, said copolyamide having a melting point measured on a PerkinElmer DSC-2 device in the range from 120° C. to 240° C.; (b) a secondfilm section, said section being a section of a film having surfacecomprising a copolyamide as defined in subparagraph (a) above; and, (c)at least one heat weld between the copolyamide surfaces of said firstand second sections thereby forming said articles for packaging foodproducts, said article being capable of withstanding, without sufferingdamage to said heat weld, heat treatment at temperatures from 70° C. to120° C. for at least 10 minutes.

Of general interest are the disclosures of U.S. Pat. No. 3,355,319issued Nov. 28, 1967 to Rees for "Self-Supporting Film With AHeat-Sealable Coating Of An Ionic Copolymer Of An Olefin And CarboxylicAcid With Metal Ions Distributed Throughout" and U.S. Pat. No. 3,845,163issued Oct. 29, 1974 to Murch for "Blends of Polyamides and IonicCopolymer". Both of these patents are assigned to E. I. du Pont deNemours and Company, and relate to metal salt neutralized copolymers ofan alpha-olefin having the formula RHC═CH₂ wherein R is H or C₁ to C₈alkyl and an alpha,beta-ethylenically unsaturated carboxylic acid. Suchmaterials are marketed by du Pont under the name, Surlyn (™).

The present invention is directed to multiple-layer cook-in film fromwhich packages, such as casings or bags, can be made which exhibitimproved heat seal strength, yet still retain at least some foodadherence characteristics and often exhibit similar or improved foodadherence characteristics as compared to known films having a Surlynfood contact surface that adheres to a food product during cook-in, suchas some of the films described in U.S. Pat. No. 4,606,922 and some ofthe films described in U.S. Pat. No. 4,469,742, both of which arediscussed above. The multilayer film structure of the present inventionhas a "sealing and food contact" layer, more preferably has the minimalstructure: (sealing and food contact layer)/(barrier layer), in oneadvantageous embodiment has the minimal structure: (sealing and foodcontact layer)/(barrier layer)/(abuse layer), and in anotheradvantageous embodiment has the minimal structure: (sealing and foodcontact layer)/(shrink layer)/( barrier layer)/(abuse layer), suchcomposite structures being advantageous to achieve the desired compositeproperties of the packaging film.

SUMMARY OF THE INVENTION

Accordingly, there is provided a multiple-layer, cook-in film having afirst food contact layer having been subjected to an energetic radiationsurface treatment, said first layer functioning as an adhering layer,and wherein said first layer comprises a polyamide composition. In anadvantageous embodiment, the composition is selected from (I) apolyamide or (II) a blend comprising about 60% up to about 100% byweight polyamide with about 40% down to about 0% by weight (i)polyethylene, or (ii) copolymer of an alpha-olefin having the formulaRHC═CH₂ wherein R is H or C₁ to C₈ alkyl and an alpha,beta-ethylenicallyunsaturated carboxylic acid.

Also, there is provided a method for improving the heat seal strength ofmultiple layer, cook-in film having a first food contact layer (a) thatfunctions as an adhering layer, said method comprising (1) providing forsaid first layer (a) with a polyamide composition (2) extruding saidfirst layer into a multiple layer film, and (3) prior to or after theextruding of said first layer (a), subjecting said first layer (a) to anenergetic radiation surface treatment. In an advantageous embodiment,said polyamide composition in said first layer (a) is provided by (I) apolyamide or (II) a blend comprising by weight about 60% up to about100% polyamide with about 40% down to about 0% (i) polyethylene, or (ii)copolymer of an alpha-olefin having the formula RHC═CH₂ wherein R is Hor C₁ to C₈ alkyl and an alpha,beta-ethylenically unsaturated carboxylicacid.

DETAILED DESCRIPTION OF THE INVENTION The First Food Contact Or AdheringLayer

The cook-in film may be made from any of a variety of multi-layercook-in packaging films so long as the first food contact layer of thefilm comprises a polyamide composition. In an advantageous embodiment,the composition in said first layer is (I) a polyamide or (II) a blendcomprising about 60% up to about 100% by weight polyamide with about 40%down to about 0% by weight (i) polyethylene, or (ii) copolymer of analpha-olefin having the formula RHC═CH₂ wherein R is H or C₁ to C₈ alkyland an alpha,beta-ethylenically unsaturated carboxylic acid. Preferablywhen there is present in the polyamide composition a component that is acopolymer of olefin and carboxylic acid, the olefin is ethylene and thecarboxylic acid is acrylic acid or methacrylic acid.

The first food contact layer desirably has a thickness of about 0.2 to1.0 mils (about 5 to 25 micrometers) for a suitable multi-layer cook-inpackaging film. The food contact layer to bonds or adheres to thecontained meat product during cook-in, thereby preventing cook-out offluids from the contained meat product. Thus, the food contact layer isalternatively referred to as the adhering layer.

The food contact layer must be subjected to an energetic radiationsurface treatment, including, but not limited to corona discharge,plasma, flame, ultraviolet, and high energy electron treatment. While itis not known for sure and thus it is not intended to limit the invention"causally" thereby, it is believed the energetic radiation surfacetreatment "causes" the food adhering characteristics. For instance, thefood contact layer may be selectively irradiated with high energyelectrons which advantageously may be accomplished during irradiation ofthe overall multi-layer film structure for cook-in integrity, as furtherdiscussed below. A suitable radiation dosage of high energy electrons isin the range of up to about 12 megarads (MR), more preferably about 2-9MR. Radiation dosages are referred to herein in terms of the radiationunit "RAD", with one million RADS or a megarad being designated as "MR".

The polyamides employed in the first food contact layer of the films ofthis invention are well known in the art and embrace those resinscommonly designated as nylons. Typically, in the conventional method,some polyamide resins are made by condensation of equimolar amounts of asaturated dicarboxylic acid containing from about 2 to 10 carbon atomswith an alkylene diamine, in which the alkylene group contains fromabout 2 to 10 carbon atoms. Excess diamine may be used, thereby givingan excess of amine end groups over carboxyl end groups in the polyamide.Other polyamide resins are polymerized by addition reactions of ringcompounds that contain both acid and amine groups on the monomer.Examples of suitable polyamides include, but are not limited to,polycaprolactam (nylon 6), (nylon 6/9), (nylon 6/10), thepolycondensation product of hexamethylenediamine and a 12-carbon dibasicacid (nylon 6/12), the polymerization product of lauric lactam ofcyclododecalactam with 11 methylene units between the linking --NH--CO--groups in the polymer chain (nylon 12), the polyaddition product of themonomer 11-aminoundecanoic acid (nylon 11), polyhexamethylene adipamide(nylon 66), (nylon 69), polyhexamethylene sebacamide (nylon 610), and(nylon 612). It is also possible to use in this invention polyamidesprepared by the copolymerization of two of the above polymers orterpolymerization of the above polymers or their components. A verydesirable nylon is Versamid (TM), which is a nylon 12 supplied byChemische Werke Huls AG, Germany. Also, Rilsan® nylon 11 or Rilsan®nylon 12, supplied by Atochem, Inc., Polymers Division, Glen Rock, N.J.,may be advantageously employed. Also, Grilamid (™) nylon 12 from EmserIndustries, Sumter, S.C. may be advantageously employed.

Suitable "polyethylenes" employed in the first food contact layer (or inanother layer as discussed below) are the families of resins obtained bysubstantially polymerizing the gas ethylene, C₂ H₄. By varying thecomonomers, catalysts and methods of polymerization, properties such asdensity, melt index, crystallinity, degree of branching andcross-linking, molecular weight and molecular weight distribution can beregulated over wide ranges. Further modifications are obtained by otherprocesses, such as halogenation, and compounding additives. Lowmolecular weight polymers of ethylene are fluids used as lubricants;medium weight polymers are waxes miscible with paraffin; and the highmolecular weight polymers are resins generally used in the plasticsindustry. Polyethylenes having densities ranging from about 0.900 g/ccto about 0.935 g/cc, more preferably to about 0.928 g/cc, are called lowdensity polyethylenes (LDPE), while those having densities from about0.936 g/cc to about 0.940 g/cc are called medium density polyethylenes(MDPE), and those having densities from about 0.941 g/cc to about 0.965g/cc and over are called high density polyethylenes (HDPE). The older,classic low density types of polyethylenes are usually polymerized athigh pressures and temperatures whereas the older, classic high densitytypes are usually polymerized at relatively low temperatures andpressures.

The term "linear low density polyethylene" (LLDPE) as used herein for atype of polyethylene employed in the first food contact layer (oranother layer) refers to the newer copolymers of ethylene with one ormore comonomers selected from C₄ to C₁₀ alpha olefins such as butene-1,pentene-1, hexene-1, octene-1, etc. in which the molecules thereofcomprise long chains with few side chains or branches achieved by lowpressure polymerization. the side branching which is present will beshort as compared to non-linear polyethylenes. The molecular chains of alinear polymer may be intertwined, but the forces tending to hold themolecules together are physical rather than chemical and thus may beweakened by energy applied in the form of heat. Linear low densitypolyethylene has a density in the range from about 0.911 g/cc to about0.935 g/cc, more pre-ferably in the range of from about 0.912 g/cc toabout 0.928 g/cc for film making purposes. The melt flow index of linearlow density polyethylene generally ranges from between about 0.1 toabout 10 grams per ten minutes and preferably between from about 0.5 toabout 3.0 grams per ten minutes. Linear low density polyethylene resinsof this type are commercially available and are manufactured in lowpressure vapor phase and liquid phase processes using transition metalcatalysts. LLDPE is well known for its structural strength andanti-stress-cracking properties. Also, very low density linear lowdensity polyethylenes (VLDPE) may be employed, and such have a densityfrom about 0.910 g/cc to about 0.860 g/cc, or even lower.

The term "ethylene vinyl acetate copolymer" (EVA) as used herein for atype of polyethylene refers to a copolymer formed from ethylene andvinyl acetate monomers wherein the ethylene derived units in thecopolymer are present in major amounts and the vinyl acetate derivedunits in the copolymer are present in minor amounts. EVA is known notonly for having structural strength, as LLDPE does, but also it is knownfor providing excellent adhesion to an adjacent layer, which maydecrease or even obviate the need for an "adhesive".

The term "ethylene-methylacrylate copolymer" (EMA) as used herein for atype of polyethylene, refers to a copolymer formed from ethylene andmethylacrylate monomers.

The term "ethylene-ethylacrylate copolymer" (EEA) as used herein for atype of polyethylene, refers to a copolymer formed from ethylene andethylacrylate monomers.

The term "ethylene butyl acrylate copolymer" (EBA) as used herein for atype of polyethylene, refers to a copolymer formed from ethylene andbutyl acrylate monomers.

Blends of all families of polyethylenes, such as blends of EVA, EMA,EEA, EBA, VLDPE, and LLDPE, may also be advantageously employed.

The copolymer of an alpha-olefin having the formula RHC-CH₂ wherein R isH or C₁ to C₈ alkyl and an alpha,beta-ethylenically unsaturatedcarboxylic acid which may be employed in the first food contact layer ofthe films of this invention representatively may be one of the Primacor(™) polymers, supplied by Dow Chemical Company, Midland, Mich. Primacoris produced by the free radical copolymerization of ethylene and acarboxylic acid comonomer therefor such as acrylic acid or methacrylicacid. A very suitable Primacor polymer is Primacor 1410. Also, thecopolymer of an alpha-olefin having the formula RHC═CH₂ wherein R is Hor C₁ to C₈ alkyl and an alpha,beta-ethylenically unsaturated carboxylicacid may be metal salt neutralized. Thus, the copolymer may be anionomer. Representatively, such an ionomeric material is commerciallyavailable as Surlyn (™) from the E. I. du Pont de Nemours Company ofWilmington, Del., and is described in detail in U.S. Pat. No. 3,355,319and U.S. Pat. No. 3,845,163, both of which are cited above.

As discussed above, the first food contact layer of the film comprises apolyamide composition; and in an advantageous embodiment the compositionis selected from (I) a polyamide or (II) a blend comprising about 60% upto about 100% by weight polyamide with about 40% down to about 0% byweight (i) polyethylene, or (ii) copolymer of an alpha-olefin having theformula RHC═CH₂ wherein R is H or C₁ to C₈ alkyl and analpha,beta-ethylenically unsaturated carboxylic acid.

As further illustrated in Example I below, in the embodiments involvingblends comprising polyamide with (ii) copolymer, those blends havingabout 55% or less polyamide exhibit poor heat-seal strength.Furthermore, there is a trade off in choosing a polyamide free of thecopolymer versus choosing a blend of polyamide with the copolymer forthe first layer of the film of the present invention, although all suchfilms exhibit a heat-seal strength superior to those known films havinga first (sealing-adhering-food contact) layer of Surlyn copolymer ofolefin and carboxylic acid. When the first layer is a polyamide free ofthe copolymer, heat-seal strength is comparable to when the first layeris a blend, as is further illustrated in Example I below. On the otherhand, when the first layer is a polyamide free of the copolymer, foodadherence characteristics are still retained but they are inconsistentas compared to when the first layer is a blend, as is furtherillustrated in Example II below. Accordingly, in this embodiment, thefirst layer desirably comprises by weight about 60% up to about 100%polyamide and about 40% down to about 0% copolymer. More desirably, thefirst layer comprises by weight about 70% to about 90% polyamide andabout 30% to about 10% copolymer.

In the embodiment wherein the first food contact layer is a blendcomprising polyamide with (i) polyethylene or with characteristicsobtained by stretching and substantially immediately cooling a resinousthermoplastic polymeric material which has been heated to a temperaturewithin its orientation temperature range so as to revise theintermolecular configuration of the material by physical alignment ofthe crystallites and/or molecules of the material to improve certainmechanical properties of the film such as, for example, shrink tensionand orientation release stress. Both of these properties may be measuredin accordance with ASTM D 2838-81. When the stretching force is appliedin one direction uniaxial orientation results. When the stretching forceis simultaneously applied in two directions biaxial orientation results.The term "oriented" is also herein used interchangeably with the term"heat shrinkable" with these terms designating a material which has beenstretched and set by cooling while substantially retaining its stretcheddimensions. An oriented (i.e. heat shrinkable) material will tend toreturn to its original unstretched (unextended) dimensions when heatedto an appropriate elevated temperature.

Returning to the basic process for manufacturing the film as discussedabove, it can be seen that the film, once coextruded and initiallycooled to by, for example, cascading water quenching, is then reheatedto within its orientation temperature range and oriented by stretching.The stretching to orient may be accomplished in many ways such as, forexample, by "blown bubble" techniques or "tenter framing". Theseprocesses are well known to those in the art and refer to orientationprocedures whereby the material is stretched in the cross or transversedirection (TD) and/or in the longitudinal or machine direction (MD).After being stretched, the film is quickly quenched while substantiallyretaining its stretched dimensions to cool the film rapidly and thus setor lock-in the oriented molecular configuration.

Of course, if a film having little or no orientation is desired, e.g.non-oriented or non-heat shrinkable film, the film may be formed from anon-orientable material or, if formed from an orientable material may be"hot blown". In forming a hot blown film the film is not cooledimmediately after extrusion or coextrusion but rather is first stretchedshortly after extrusion while the film is still at an elevatedtemperature above the orientation temperature range of the material.Thereafter, the film is cooled, by well-known methods. Those of skill inthe art are well familiar with this process and the fact that theresulting film has substantially unoriented characteristics. Othermethods for forming unoriented films are well known. Exemplary, is themethod of cast extrusion or cast coextrusion which, likewise, is wellknown to those in the art.

Whichever film has been made (the non-oriented molecular configurationor the stretch-oriented molecular configuration), it may then besubjected to an energetic radiation surface treatment, which isadvantageously provided by a high energy electron treatment. Forinstance, it may be irradiated, for example by guiding it through thebeam of an electron accelerator to receive a radiation dosage up toabout 12 megarads (MR), move preferably a dosage in the range of about2-9 megarads (MR), and then it may be stored in rolls and utilized topackage a wide variety of items. In this regard, the product to bepackaged may first be enclosed in the material by heat sealing the filmto itself where necessary and appropriate to form a pouch or bag andthen inserting the product therein. If the material was manufactured by"blown bubble" techniques the material may still be in tubular form orit may have been slit and opened up to form a sheet of film material.Alternatively, a sheet of the material may be utilized to over-wrap theproduct. These packaging methods are all well known to those of skill inthe art.

If the material is of the heat shrinkable type, then thereafter theenclosed product may be subjected to elevated temperatures, for example,by passing the enclosed product through a hot air or hot water tunnel.This causes the enclosing heat shrinkable film to shrink around theproduct to produce a tight wrapping that closely conforms to the contourof the product. As stated above, the film sheet or tube may be formedinto bags or pouches and thereafter utilized to package a product. Inthis case, if the film has been formed as a tube it may be preferablefirst to slit the tubular film to form a film sheet and thereafter formthe sheet into bags or pouches. Such bag or pouch forming methods,likewise, are well known to those of skill in the art.

The above general outline for manufacturing of films is not meant to beall inclusive since such processes are well known to those in the art.For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039;4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of thesepatents are generally representative of such processes and are herebyincorporated by reference.

Alternative methods of producing films of this type are known to thosein the art. One well-known alternative is the method of forming amulti-layer film by an extrusion coating rather than by an extrusion orcoextrusion process as was discussed above. In extrusion coating a firsttubular layer is extruded and thereafter an additional layer or layersis sequentially coated onto the outer surface of the first tubular layeror a successive layer. Exemplary of this method is U.S. Pat. No.3,741,253. This patent is generally representative of an extrusioncoating process and is hereby incorporated by reference.

Many other process variations for forming films are well known to thosein the art. For example, multiple layers may be first coextruded withadditional layers thereafter being extrusion coated thereon. Or twomulti-layer tubes may be coextruded with one of the tubes thereafterbeing extrusion coated or laminated onto the other. The extrusioncoating method of film formation is preferable to coextruding the entirefilm when it is desired to subject one or more layers of the film to atreatment which may be harmful to one or more of the other layers.Exemplary of such a situation in a case where it is desired to irradiatewith high energy electrons one or more layers of a film containing abarrier layer comprised of one or more copolymers of vinylidene chloride(i.e. saran), such as of vinylidene chloride and vinyl chloride or suchas of vinylidene chloride and methyl acrylate. In other words, thebarrier layer includes a saran layer in addition to or instead of anEVOH layer. Those of skill in the art generally recognize thatirradiation with high energy electrons is generally harmful to suchsaran barrier layer compositions, as irradiation may degrade anddiscolor saran, making it turn brownish. Thus, if full coextrusionfollowed by high energy electron irradiation of the multilayer structureis carried out on a film having a barrier layer containing a saranlayer, the irradiation should be done at low levels with care.Alternatively, this situation may be avoided by using extrusion coating.Accordingly, by means of extrusion coating, one may first extrude orcoextrude a first layer or layers, subject that layer or layers to highenergy electron irradiation and thereafter extrusion coat the saranbarrier layer and, for that matter, other later layers (which may or maynot have been irradiated) sequentially onto the outer surface of theextruded previously irradiated tube. This sequence allows for the highenergy electron irradiative treatment of the first and later layer orlayers without subjecting the saran barrier layer to the harmfuldiscoloration effects thereof.

Thus, as used herein the term "extrusion" or the term "extruding" isintended to include coextrusion, extrusion coating, or combinationsthereof.

One Embodiment (A Film For A Food Casing)

One representative embodiment of the invention is a multi-layer film fora tubular food casing having an outside nylon layer over one or moreother layers, and having a Surlyn and nylon blend as the food contactinner surface, for example the structure: (food contact and inside)blend of Surlyn and nylon/adhesive/(outside) nylon. The Surlyn and nylonblend food-contact inner surface functions as a food adhering material.Nylon 6 or nylon 66 is preferred for the outside layer, as thesematerials not only serve as a fluid barrier, such as an oxygen barrier,but also impart high stuffing strength to the casing. Accordingly, theoutside nylon 6 or nylon 66 layer functions both as a barrier layer andas an abuse layer. In addition to irradiatively treating the foodcontact layer, the adhesive layer desirably is also irradiated, tocross-link it for enhanced cook-in structural integrity. Any of thevarious adhesives well known in the art of film making may be employed.Some representative adhesives that are suitable are those adhesives thatcomprise a chemically modified polyolefin selected from the groupconsisting of ethylene-vinyl acetate copolymer, high densitypolyethylene and rubber modified high density polyethylene, eachchemically modified by the provision of functional groups which willform a strong bond to the adjacent layer, herein nylon, under heat andpressure of coxtrusion, such strong bonding being representatively shownin U.S. Pat. No. 4,233,367. The preferred adhesives are the acidanhydride grafted polyethylenes such as the Plexar (™) adhesives (mostpreferably Plexar-3 adhesive), supplied by Chemplex Company of RollingMeadows, Ill., which adhesives are further discussed herein below. Theblend of Surlyn and nylon in the first food contact or adhering layer ismade as discussed above.

Another Embodiment (A Heat Shrinkable Film)

Another representative embodiment of the invention is a compositetubular film having the multilayer structure: (inside) A/B/C/D/C/E(outside) where A is primarily a food contact layer, B is primarily ashrink layer, C is primarily an adhesive layer, D is primarily a barrierlayer, and E is primarily an abuse layer. This film, when in a tubularconfiguration, is especially suited for bag making. The material oflayer A (the first food contact layer) is made as discussed above and isrepresentatively a blend of a polyamide with an ionomer of a metal saltneutralized copolymer of an alpha-olefin having the formula RHC═CH₂wherein R is H or C₁ to C₈ alkyl, preferably ethylene, and analpha,beta-ethylenically unsaturated carboxylic acid, preferably acrylicacid or methacrylic acid. Accordingly, this first layer A isalternatively referred to as the adhering layer. Layer B, the secondlayer, being a shrink layer, is typically melt bonded to the first layerand is representatively an ethylene homopolymer or copolymer such as anethylene-vinyl acetate copolymer (EVA), an ethylene-butyl acetatecopolymer (EBA), a linear low density polyethylene (LLDPE), a blend ofEVA and LLDPE, a blend of EBA and LLDPE, very low density polyethylene(VLDPE), Plexar, or a blend of Plexar and LLDPE. The term "shrink layer"is intended to refer to the shrink controlling layer that initiatescompatible shrinkage, i.e. during hot water immersion, of the overallmultilayer structure. The relative thickness of the shrink layer isselected as being sufficient relative to that of the overall filmthickness such that the shrink temperature of the shrink layer controlsthe shrinkage of the entire multi-layer film, when oriented. Barrierlayer D is representatively composed of Saran (a vinylidene chloridecopolymer) layer, a hydrolyzed ethylene-vinyl acetate copolymer (EVOH)layer, or both a saran layer and an EVOH layer. When the barrier layeris composed of both a saran layer and an EVOH layer, a suitable adhesivemay be employed between them. Adhesive interlayers C are melt bondedadjacent the barrier layer to provide delamination resistance of thebarrier layer in the tubular film under cook-in conditions. The adhesivemay be any of the various adhesives well known in the art of filmmaking. Representatively, a suitable adhesive is composed of a copolymeror a homopolymer of olefin (preferably that is cross-linkable such as byirradiation) that has been modified to provide functional groups with arelatively strong affinity for adjacent layer, i.e. the barriermaterial. Preferably, the adhesive is Plexar (™) adhesive commerciallyavailable from the Enron Chemical Company. Abuse layer E isolates thebarrier layer from adverse moisture contact and representatively is anethylene-vinyl acetate copolymer having a vinyl acetate content of up toabout 25% preferably about 5-12%, more preferably about 6%, or a blendthereof with LLDPE or VLDPE. All layers within the film are typicallymelt bonded to the respective adjacent layers. Representatively, thefilm will have an overall thickness prior to orientation of about 10-30mils (about 254-762 micrometers), food contact layer A will have athickness of about 2-6 mils (about 51-152 micrometers), shrink layer Babout 4-8 mils (about 102-203 micrometers), adhesive layers C about0.25-1.5 mils (about 6.3-76 micrometers) each, barrier layer D about0.75-2 mils (about 19-102 micrometers), and abuse layer E about 3-8 mils(about 76-203 micrometers).

In the embodiment wherein the film of the invention is a shrink filmhaving a barrier layer comprising EVOH, the film may be made by atubular process similar to that described for the Oberle et al patent,cited above, wherein the tubular film is fully coextruded, i.e. alllayers are simultaneously coextruded, using the conventional blownbubble technique. Full coextrusion is advantageous in that all layers ofthe multi-layer film are directly melt joined for enhanced interlayerstrength under cook-in conditions. After cooling, the coextruded tube isflattened and then guided through an ionizing radiation field, forexample through the beam of a high energy electron accelerator toreceive a radiation dosage in the range of up to about 12 megarads (MR).Irradiation via this high energy electron treatment of the overallmulti-layer film structure achieves the required energetic radiationtreatment of the food contact layer. As the film in this embodiment is aheat shrinkable film, in general, irradiation should be sufficient tocross-link the irradiatively cross-linkable layers of the film toincrease strength of the shrink layer without substantially diminishingelongation properties, and to provide delamination resistance of thefilm during cook-in conditions. After irradiation, the tube is then fedinto a hot water tank having water at about 190°-212° F. (88°-100° C.)to soften the film for orientation; then it passes through pinch rollsand is inflated into a bubble and stretched to a point where the filmthickness is representatively 2 mils (about 51 micrometers). Suitablethickness will range from about 1-4 mils (about 25-102 micrometers) witha stretch ratio of about 5-12:1, which will impart a shrink capacity ofup to about 55% biaxial free shrinkage at 185° F. (85° C.) (by ASTMD2732). As the stretched bubble emerges from the hot water tank it coolsrapidly in the air and then is collapsed and rolled up into flattenedtubing. It is from this tubing of this final oriented thickness thatbags are made as discussed above.

In use, bags are made from the heat-shrinkable film, as discussed above,to form either end-seal or side-seal bags. Eventually, the bags areloaded with a food product, vacuumized and sealed, and subjected tocook-in treatment in near boiling water. During this food treatment, (1)bags (a) maintain good seal integrity, (b) do not delaminate and (c)heat shrink to form a neatly packaged pretreated food product, and (2)the first food contact layer (the inner layer of the bag) bindinglyadheres to the contained food product to enhance weight yield of thecooked food product.

More particularly, the second or shrink layer B may be an ethylenehomopolymer or copolymer such as linear low density polyethylene(LLDPE), low density polyethylene (LDPE), very low density polyethylene(VLDPE), ethylene-vinyl acetate copolymer (EVA), a blend of VLDPE orLLDPE and EVA, ethylene-butyl acrylate copolymer (EBA), a blend of EBAand VLDPE or LLDPE, ethylene-methylacrylate copolymer (EMA), or a blendof EMA and VLDPE or LLDPE. Also, the second layer may be an acidanhydride grafted polyethylene such as Plexar or may be a blend ofPlexar and LLDPE. Since Plexar may also be employed as the adhesivelayer, either for this embodiment comprising a shrink film or for theembodiment discussed above comprising a film having the structure: (foodcontact or inside) blend of Surlyn and nylon/adhesive/(outside) nylon,further comments about Plexar are in the paragraph below discussing theadhesive layer. When the shrink layer is EVA, preferably, the shrinklayer is composed of EVA having a vinyl acetate content in a range of upto about 25%, more preferably about 6-12%, most preferably about 6%,with the orientation temperature generally decreasing and shrinkcapacity increasing as the vinyl acetate content is increased. However,the melt temperature of EVA tends to decrease as the vinyl acetatecontent increases so that a maximum content of up to about 12% isdesirable with a melting temperature of about 95° C. Irradiativecross-linking corresponding to a dosage of up to about 12 MR providessufficient cross-linking in the shrink layer to enable production of thetubular film and orientation by the blown bubble technique at economicproduction rates.

More particularly, layer D (the barrier layer) may be composed of alayer comprising vinylidene chloride copolymer, composed of a layercomprising hydrolyzed ethylene-vinyl acetate copolymer (EVOH),preferably hydrolyzed to at least about 50%, most preferably to greaterthan about 99%, or composed of both a layer comprising vinylidenechloride copolymer and a layer comprising EVOH. Preferably, the barrierlayer is composed of a layer comprising EVOH, and the mole percent ofvinyl acetate prior to hydrolysis should be at least about 29%, sincefor lesser amounts the effectiveness of the hydrolyzed copolymer as abarrier to fluids such as gas is substantially diminished. It is furtherpreferred that the EVOH copolymer have a melt flow being generallycompatible with that of the other components of the multilayer film,preferably in the range of about 3-10 (melt flow being determinedgenerally in accordance with ASTM D1238). The gas of main concern isoxygen and transmission is considered to be sufficiently low, i.e. thematerial is relatively gas impermeable, when the transmission rate isbelow 70 cc/m² /mil thickness/24 hours/atms, as measured according tothe procedures of ASTM Method D-1434. The barrier layer of themultilayer film of this shrink film embodiment of the present inventionhas a transmission rate below this value. EVOH is advantageouslyutilized in the film of the invention since irradiative high energyelectron treatment of the fully coextruded film does not degrade an EVOHbarrier layer, as could be the case for a vinylidene chloride copolymerbarrier layer. If a vinylidene chloride copolymer is employed instead ofor together with EVOH as the barrier layer, then the necessary energeticradiation treatment for the food contact layer preferably should takeplace prior to application of the saran layer to avoid degradationthereof, which application may be achieved by well known extrusioncoating methods, as discussed above.

The adhesive interlayers C may be any of the various adhesives wellknown in the art of film making. More particularly, the adhesiveinterlayers C melt bonded adjacent the barrier layer are composedgenerally of a copolymer or homopolymer of olefin which is modified bythe provision of functional groups having a strong affinity for thebarrier layer and which will form a strong bond under the heat andpressure of coextrusion. Preferably, it is cross-linkable, such as byirradiation. Preferably, the adhesive is one of the Plexar (TM)adhesives commercially available from the Chemplex Company of RollingMeadows, Ill. Generally, Plexar adhesive is composed of an acidanhydride grafted polyethylene being irradiatively cross-linkable.Plexar adhesives are described in detail in U.S. Pat. Nos. 4,087,587 and4,087,588. Plexar-2 adhesive may generally characterized as an adhesiveof the type comprising blends of a graft copolymer of a high densitypolyethylene and at least one unsaturated, fused ring, carboxylic acidanhydride, blended with one or more resin copolymers of ethylene and anethylenically unsaturated ester. Plexar-3 is preferred which comprisesblends of a graft copolymer of a high density polyethylene and at leastone unsaturated fused ring carboxylic acid anhydride, blended with apolyethylene resin of one of more homopolymers of ethylene, copolymersof ethylene and an alpha-olefin or any or all of these. Another suitableadhesive is Admer LF500 (™) commercially available from the MitsuiCompany which comprises a low density polyethylene chemically modifiedwith phthalic acid to an extent sufficient for the above statedfunction. The adhesive layer is cross-linked, preferably by irradiation.Another suitable adhesive is Bynel CXA E-162, supplied by du Pont. It isan EVA having a vinyl acetate content of about 18% blended with agrafted polyethylene.

More particularly, the outer layer E may be an abuse layer, provided toisolate the preferred EVOH barrier layer D from moisture contact andthereby to prevent degradation in barrier properties. The abuse layermay be composed preferably of an ethylene homopolymer or copolymer suchas EVA or a blend of EVA and LLDPE as discussed above for the foregoingshrink layer. When the abuse layer is composed of EVA, preferably thevinyl acetate content is up to about 25%, more preferably about 5-12%,most preferably about 6%. Alternatively, the outer abuse layer may bethe same as the sealing layer A, this configuration being appropriatefor form/fill/seal packaging wherein heat sealing is done on overlappededge portions of a sheet of film.

The resins or basic polymeric materials fed into the extruders to makethe film of the present invention are widely available and can bepurchased from any of a number of suppliers, for example thoseidentified in trade publications such as Modern Plastics Encyclopedia.

The following examples illustrate the preferred embodiments of theinvention. It is not intended to limit the invention thereby.

EXAMPLES

In the data Tables, a number of samples are presented. Bags wereprepared of the various film types indicated in the Tables by theconventional tubular, blown bubble, coextrusion method substantially asdescribed above for the 6-layer shrink film of the structure:A/B/C/D/C/E, wherein A is the first food contact layer, B is the shrinklayer, layers C are the adhesive layers, D is the barrier layer and E isthe outside layer. Only those compositions which could be successfullyprocessed by this method into film heat sealed in bag form are shown inthe Tables. In all samples, layers C are Plexar-3, layer D is EVOH, andlayer E is a blend of 80% EVA and 20% LLDPE by weight. Layers A and Bvery as designated in the Tables. The Surlyn (™) employed in Layer A wassupplied by E. I. du Pont de Nemours. The Primacor (™) employed in LayerA was Primacor 1410 supplied by Dow Chemical Company. The nylon employedin Layer A was Versamid (™), which is marketed by supplier ChemischeWerke Huls, Germany, and is a nylon 12. The EVA employed in Layer B, inone of the samples was Petrothene® NA 295 supplied by U.S. IndustrialChemicals Company, Cincinnati, Ohio, a division of National Distillersand Chemical Corp. The Plexar employed in Layer B in several of thesamples was Plexar-3, supplied by Enron Chemical Company (formerlyChemplex). The LLDPE employed in layer A and/or Layer B of several ofthe samples was Dowlex (™) 2045.03 supplied by Dow Chemical Company ofMidland, Mich. The percentages of these materials employed are based on% by weight. In all samples, the film was irradiated in an electron beamunder a dosage of about 4 megarads (MR). The film samples had multilayerdimensions prior to orientation of about 4 mils (about 102 micrometers)for the food contact layer, about 5 mils (about 127 micrometers) for theshrink layer, about 1.25 mils (about 132 micrometers) for the firstadhesive layer, about 1.25 mils (about 132 micrometers) for the barrierlayer, about 1.25 mils (about 132 micrometers) for the second adhesivelayer, and about 5 mils (about 127 micrometers) for the outside layer.Tubular film samples following irradiation were biaxially orientedcorresponding to a stretch ratio of about 7.5:1 for a final overall filmthickness of about 2.4 mils (about 61 micrometers). The film was madeinto end seal bags.

EXAMPLE I Heat Seal Strength

Two sets of bags were used in this example, so that for each samplenumber as indicated in Table I below, there would be a first set of bagsfor testing at 182° F. (83° C.) and a second set for testing at 73° F.(23° C.) (room temperature).

All bag samples were heat sealed at one end on conventional equipmentwell known in the art of heat sealing of tubing, and a mouth end of eachbag was left open.

However, prior to heat sealing the first set of bags for the 83° C.test, the inside surface of the heat seal area was smeared with a thinlayer of peanut oil. Peanut oil was used to simulate the fats and oilspresent in many food products.

Each bag of both sets was then clamped in a fixture provided with ahose. The open mouth end of each bag was clamped around the hose. Airwas pumped through each hose whereby each bag was inflated to the sameinitial pressure. Then, for the first set of bags, each fixture loweredthe heat sealed end of each inflated bag approximately 5 cm into a hotwater bath at 83° C. For the second set of bags, each fixture retainedeach inflated bag in air at 23° C. and two sides of each bag wererespectively restrained by two metal plates spaced about 10 cm apart.For each bag of both sets, the pressure was increased via the hose atthe rate of 1 inch of water pressure (2491 dynes/cm²) per second tillthe heat seal for that bag either leaked or burst open at the IOWP(inches of water pressure) designated in Table I below.

                  TABLE I                                                         ______________________________________                                        Film                         IOWP   IOWP                                      Type   Food                  at Burst                                                                             at Burst                                  Sample Contact               or Leak                                                                              or Leak                                   Number Layer A    Layer B    (83° C.)                                                                      (23° C.)                           ______________________________________                                        1      90% nylon 12                                                                  10% Surlyn Plexar-3   76     182                                       2      80% nylon 12                                                                  20% Surlyn Plexar-3   73     204                                       3      70% nylon 12                                                                  30% Surlyn Plexar-3   65     193                                       4      75% nylon 12                                                                             80% EVA                                                            25% Surlyn 20% LLDPE  65.9   161                                       5      75% nylon 12                                                                             50% Plexar-3                                                       25% Surlyn 50% LLDPE  67.7   172                                       6      75% nylon 12                                                                  25% Surlyn Plexar-3   66.9   186                                       7      75% nylon 12                                                                             Plexar-3   61.9   199                                              25% Primacor                                                           8      nylon 12   Plexar-3   75.8   168                                       9      75% nylon 12                                                                             Plexar-3   73     163                                              25% LLPDE                                                              10     Surlyn     Plexar-3   40     NOT                                       (Control)                           TESTED                                    11     55% nylon 12                                                                  45% Surlyn Plexar-3   28.8   196                                       12     30% nylon 12                                                                  70% Surlyn Plexar-3   31.4   200                                       ______________________________________                                    

Sample 10 was a control sample wherein food contact layer A contained nonylon 12, but rather was 100% Surlyn. As discussed above, the prior artteaches 100% Surlyn for the food contact-meat adhering layer. But as canbe seen from the table, control sample 10 had a seal strength of only 40IOWP (at the hot temperature of 82° C.), whereas the blends of 75% to90% nylon 12 by weight either with Surlyn or with Primacor exhibited amuch improved seal strength ranging from about 70 to about 76 IOWP (atthe hot temperature of 83° C.). Additionally, it is noted that forsample 8, wherein layer A was only nylon 12 free of any blending withLLDPE or with a copolymer such as Primacor or Surlyn, the 83° C. sealstrength was about 76 IOWP, which is comparable to the 83° C. sealstrength of the blends and much better than the 40 IOWP 83° C. sealstrength of control sample 10. Furthermore, it can be seen from samples11 and 12 at the bottom of the table, that when layer A was a blend andthe amount of nylon 12 therein was decreased to 55% and 30%,respectively, the 83° C. seal strength was only around 29 to 30 IOWP,which is not as good as the 40 IOWP of the prior art 100% Surlyn for the"food contact-meat adhering" layer.

EXAMPLE II Binding Adherence to Food

Bags of several of the film sample types of Example I were tested forbinding adherence to a cooked-in meat product. Each bag was stuffed withchicken emulsion, vacuumized, heat-sealed, and cooked at 55° C. for 30minutes, and then at 60° C. for 30 minutes, and then at 65° C. for 30minutes, for a total of 90 minutes cooking time, followed by cooling inan ice bath. With film sample types 3 and 8, the procedure was repeated.

A quantitative comparison of the adherence level of several of thesamples in relation to that of control sample 10 (the sample wherein thefood contact layer was only Surlyn) was determined as follows. Aftercook-in and cooling, each sample was placed in the jaws of a Scotttester CRE 1000, and the force to pull the bag from the meat wasmeasured at a constant crosshead speed. (Another machine commonly usedfor such measuring is the Instron model 1122 tester.) The force to pullaway control bag film sample 10 was designated as 100%, with the forceto pull away the other sample bag films designated as a percent thereof.

All quantitatively tested samples were also qualitatively designated asgood, fair, or poor in relation to control sample 10.

                                      TABLE II                                    __________________________________________________________________________                  FIRST COOK-IN                                                                             REPEAT COOK-IN                                                    Quali-      Quali-                                                            tative                                                                             Quanti-                                                                              tative                                                                              Quanti-                                       Film          Adher-                                                                             tative Adher-                                                                              tative                                        Type  Food    ence Adher- ence  Adher-                                        Sample                                                                              Contact Proper-                                                                            ence   Proper-                                                                             ence                                          Number                                                                              Layer A ties Level  ties  Level                                         __________________________________________________________________________    Comp- PER     Poor 0%     NT*   NT                                            arison                                                                        1     90%                                                                           nylon 12                                                                      10%                                                                           Surlyn  Good 117%   NT    NT                                            2     80%                                                                           nylon 12                                                                      20%                                                                           Surlyn  Good 129%   NT    NT                                            3     70%                                                                           nylon 12                                                                      30%                                                                           Surlyn  Good 136%   Good  149%                                          4     75%                                                                           nylon 12                                                                      25%                                                                           Surlyn  Good NT     NT    NT                                            5     75%                                                                           nylon 12                                                                      25%                                                                           Surlyn  Good NT     NT    NT                                            6     75%                                                                           nylon 12                                                                      25%                                                                           Surlyn  Good NT     NT    NT                                            8     nylon 12                                                                              Fair 22%    Good  110%                                          9     75%     Good 90%    NT    NT                                                  nylon 12                                                                      25% LLDPE                                                               10    Surlyn  Good 100%   NT    NT                                            (Control)                                                                     __________________________________________________________________________     *NT = Not Tested                                                         

As also can be seen from Table II, for adherence comparison with filmshaving a known food contact layer made of Surlyn and adherencecomparison with the films of the present invention, a comparison samplecomprising a 6-layer film having a known food contact layer made ofpropylene-ethylene random copolymer (PER) was tested. Film having a PERfood contact layer is illustrative of another embodiment of the Oberleet al U.S. Pat. No. 4,469,742, mentioned above. It is well known thatPER makes a high strength heat-seal. PER, however, unlike a Surlyn foodcontact layer, does not tend to adhere to a contained meat productduring cook-in, which the 0% quantitative adherence of this comparativesample illustrates, as is noted in Table II.

Furthermore, it can be seen from Table II that for all samples whereinlayer A was a blend in accordance with one embodiment of the presentinvention, adherence was qualitatively good, as compared to controlsample 10, and percentage wise quantitatively better than control sample10 for those samples which were quantitatively tested. But for sample 8wherein layer A was 100% nylon 12 in accordance with another embodimentof the present invention, adherence was inconsistent, i.e. 22% in thefirst cook-in and 110% in th repeat cook-in, as compared to controlsample 10.

What is claimed is:
 1. A multiple-layer, cook-in film having a firstfood contact layer (a) having been subjected to an energetic radiationsurface treatment provided by a high energy electron treatment to anextent corresponding to a dosage of up to about 12 MR, said first layer(a) functioning as an adhering layer, and wherein said first layer (a)comprises a polyamide composition.
 2. The film of claim 1, wherein inthe polyamide composition, the polyamide selected from the groupconsisting of is nylon 6, nylon 11, nylon 12, nylon 66, nylon 610, nylon6/9, nylon 6/10, nylon 6/12, nylon 69, nylon 612, a nylon produced fromthe polycondensation or polyaddition of any of the acid or aminecompounds used to produce any of said nylons, a copolymer of any of saidnylons, a terpolymer of any of said nylons, or a mixture thereof.
 3. Thefilm of claim 1, wherein said polyamide composition is said first layer(a) is selected from (I) a polyamide or (II) a blend comprising about60% up to about 100% by weight polyamide with about 40% down to about 0%by weight of (i) a polyethylene, or (ii) a copolymer of an alpha-olefinhaving the formula RHC═CH₂ wherein R is H or C₁ to C₈ alkyl and analpha,beta-ethylenically unsaturated carboxylic acid.
 4. The film ofclaim 3, wherein in the copolymer of an olefin and a carboxylic acid,the olefin is ethylene and the carboxylic acid is acrylic acid ormethacrylic acid.
 5. The film of claim 4, wherein the copolymer is ametal salt neutralized ionomer.
 6. The film of claim 3, wherein saidpolyethylene is LDPE, MDPE, HDPE, LLDPE, VLDPE, EMA, EEA, EBA, EVA, or amixture thereof.
 7. The film of claim 1, wherein said film is selectedas comprising at least the multilayer film structure: first food contactlayer/barrier layer.
 8. The film of claim 7, wherein said film isselected as comprising at least the multilayer film structure: firstfood contact layer/barrier layer/abuse layer.
 9. The film of claim 8,wherein said film is selected as comprising at least the multilayer filmstructure: first food contact layer/shrink layer/barrier layer/abuselayer.
 10. The film of claim 1, further including additional layerscomprising:(b) an adhesive layer comprising a copolymer or homopolymerof olefin modified by the provision of functional groups with arelatively strong affinity for the following barrier layer; (c) abarrier layer comprising a nylon, a polymer or copolymer of vinylidenechloride, or a hydrolyzed ethylene-vinyl acetate copolymer.
 11. The filmof claim 1, further including additional layers comprising:(b) a secondor shrink layer, melt bonded to said first layer, which comprises anethylene homopolymer or copolymer selected from linear low densitypolyethylene (LLDPE), low density polyethylene (LDPE), ethylene-vinylacetate copolymer (EVA), medium density polyethylene (MDPE), highdensity polyethylene (HDPE), very low density polyethylene (VLDPE),ethylene-butyl acrylate copolymer (EBA), ethylene-methylacrylatecopolymer (EMA), ethylene-ethylacrylate copolymer (EEA), an acidanhydride grafted polyethylene, or a mixture thereof, further providedthat the thickness of said second layer is sufficient such that theshrink temperature of the entire multilayer film, when oriented, issubstantially controlled by the shrink temperature of said second layer;(c) a third or adhesive layer, melt bonded to said second layer, whichcomprises a copolymer or homopolymer of ethylene modified by theprovision of functional groups with a relatively strong affinity for thefollowing barrier layer; (d) a fourth or barrier layer, melt bonded tosaid third layer, which comprises a polymer or copolymer of vinylidenechloride layer, a hydroloyzed ethylene-vinyl acetate copolymer layer, orboth a polymer or copolymer of vinylidene chloride layer and ahydrolyzed ethylene-vinyl acetate layer; (e) a fifth or adhesive layerof substantially the same composition as said third layer, melt bondedto said fourth layer; and (f) a sixth or abuse layer comprising (i)LLDPE, EVA, LDPE, HDPE, MDPE, VLDPE, EBA, EMA, EEA or a mixture thereof,or (ii) the same material as said first layer.
 12. The film of claim 11,wherein said film is biaxially oriented to an extent corresponding to abiaxial free shrinkage at 185° F. (85° C.) of up to about 55%.
 13. Amethod for improving the heat seal strength of multiple layer, cook-infilm having a first food contact layer (a) that functions as an adheringlayer, said method comprising (1) providing for said first layer (a)with a polyamide composition (2) extruding said first layer into amultiple layer film, and (3) prior to or after the extruding of saidfirst layer (a), subjecting said first layer (a) to an energeticradiation surface treatment.
 14. The method of claim 13, wherein in thepolyamide composition, the polyamide selected from the group consistingof is nylon 6, nylon 11, nylon 12, nylon 66, nylon 610, nylon 6/9, nylon6/10, nylon 6/12, nylon 69, nylon 612, a nylon produced from thepolycondensation or polyaddition of any of the acid or amine compoundsused to produce any of said nylons, a copolymer of any of said nylons, aterpolymer of any of said nylons, or a mixture thereof.
 15. The methodof claim 13, wherein said polyamide composition in said first layer (a)is provided by (I) a polyamide or (II) a blend comprising by weightabout 60% up to about 100% polyamide with about 40% down to about 0% (i)polyethylene, or (ii) copolymer of an alpha-olefin having the formulaRHC═CH₂ wherein R is H or C₁ to C₈ alkyl and an alpha,beta-ethylenicallyunsaturated carboxylic acid.
 16. The method of claim 15, wherein in thecopolymer of an olefin and a carboxylic acid, the olefin is ethylene andthe carboxylic acid is acrylic acid or methacrylic acid.
 17. The methodof claim 16, wherein the copolymer is a metal salt neutralized ionomer.18. The method of claim 15, wherein said polyethylene is LDPE, MDPE,HDPE, LLDPE, VLDPE, EMA, EEA, EBA, EVA, or a mixture thereof.
 19. Themethod of claim 13, wherein said extruding provides for a filmcomprising at least the multilayer film structure: first food contactlayer/barrier layer.
 20. The method of claim 19, wherein said extrudingprovides for a film comprising at least the multilayer film structures:first food contact layer/barrier layer/abuse layer.
 21. The method ofclaim 20, wherein said extruding provides for a film comprising at leastthe multilayer film structure: first food contact layer/shrinklayer/barrier layer/abuse layer.
 22. The method of claim 13, whereinsaid extruding further includes additional layers comprising:(b) anadhesive layer comprising a copolymer or homopolymer of olefin modifiedby the provision of functional groups with a relatively strong affinityfor the following barrier layer; (c) a barrier layer comprising a nylon,a polymer or copolymer of vinylidene chloride, or a hydrolyzedethylene-vinyl acetate copolymer.
 23. The method of claim 13, whereinsaid extruding further includes additional layers comprising:(b) asecond or shrink layer, melt bonded to said first layer, which comprisesan ethylene homopolymer or copolymer selected from linear low densitypolyethylene (LLDPE), low density polyethylene (LDPE), ethylene-vinylacetate copolymer (EVA), medium density polyethylene (MDPE), highdensity polyethylene (HDPE), very low density polyethylene (VLDPE),ethylene-butyl acrylate copolymer (EBA), ethylene-methylacrylatecopolymer (EMA), ethylene-ethylacrylate (EEA), an acid anhydride graftedpolyethylene, or a mixture thereof, further provided that the thicknessof said second layer is sufficient such that the shrink temperature ofthe entire multilayer film, when oriented, is substantially controlledby the shrink temperature of said second layer; (c) a third or adhesivelayer, melt bonded to said second layer, which comprises a copolymer orhomopolymer of ethylene modified by the provision of functional groupswith a relatively strong affinity for the following barrier layer; (d) afourth or barrier layer, melt bonded to said third layer, whichcomprises a polymer or copolymer of vinylidene chloride layer, ahydrolyzed ethylene-vinyl acetate copolymer layer, or both a polymer orcopolymer of vinylidene chloride layer and a hydrolyzed ethylene-vinylacetate copolymer layer; (e) a fifth or adhesive layer of substantiallythe same composition as said third layer, melt bonded to said fourthlayer; and (f) a sixth or abuse layer comprising (i) LLDPE, EVA, LDPE,HDPE, MDPE, VLDPE, EBA, EEA, EMA, or a mixture thereof, or (ii) the samematerial as said first layer.
 24. The method of claim 23, wherein saidfilm is biaxially oriented to an extent corresponding to a biaxial freeshrinkage at 185° F. (85° C.) of up to about 55%.
 25. The method ofclaim 13, wherein said first layer has been subjected to an energeticradiation treatment provided by a high energy electron treatment to anextent corresponding to a dosage of up to about 12 MR.